Classical holograms are most commonly created by recording the complex diffraction pattern of laser light reflected from a physical object. These holograms can reconstruct images of sub-micron detail with superb quality. Ever since the early days of holography, there has been considerable interest in forming holograms of computer generated objects by computing and recording their diffraction patterns. These holograms are usually referred to as computer generated holograms, or CGH's. The computational task is a formidable one because of the enormity of the data required for good imagery. For example, a typical 10 centimeter by 10 centimeter hologram can resolve more than 10.sup.14 image points. Furthermore, no portion of the hologram surface pattern can be completely calculated until the diffraction transformation has been carried out on every one of these resolvable points. This necessitates the use of a rather large active memory; 10.sup.10 bytes for our hypothetical 10.times.10 centimeter hologram.
Even more problematic is the requirement that, for viewing over a reasonable angle, this information must be deposited into the hologram surface at a density of less than 1 pixel per micron and with about 24 bits of intensity per pixel. Many schemes have been developed for recording in a binary fashion, a process which further reduces the required pixel size.
Holograms can be composed from a multiplicity of independent object views, as was discussed in a paper by King, et al, published in Applied Optics in 1970, entitled "A New Approach to Computer-Generated Holograms." These holograms are the type discussed in this disclosure wherein they are referred to as `composite` holograms. A rather elementary but effective technique for creating composite holograms with computer generated images borrows holographic technology which was developed for other media; most notably cinematography film of physical objects. This process is discussed in a patent by K. Haines which issued in July 1982 as U.S. Pat. No. 4,339,168.
In a common embodiment of this method, many conventional views of an object are collected along a simple linear or circular trajectory. Each of these views is then processed in an optical system to build up portions of a first or storage hologram (sometimes referred to as an h1). This storage hologram bears some similarity to the drum multiplex holograms, examples of which contain fully rendered computer images.
The image from this storage hologram, as with all holographic images, is best reconstructed when the hologram is illuminated with a specific light source located at a predetermined position. Otherwise an image degradation results which is a function of the distance between the image points and the hologram surface. In order to make a hologram which is clearly discernable, even under adverse lighting conditions, one should therefore construct an image-plane hologram in which the image straddles the hologram plane.
In order to make an image-plane hologram, the image from the first hologram is used as an object which is recorded in a second hologram, which is frequently referred to as an h2. The laser light rays which constituted the object of the h1, are reconstructed (a rather unique capability of holography) by reversing the direction of the h1 reference beam. This results in the construction of a 3D image of the original object, albeit a pseudoscopic or inside-out image, in a space which is now accessible for placement of an h2. The h2 is located on a plane within the image volume.
With many image-plane holograms, the view-ability is further enhanced under polychromatic (white light) illumination with the elimination of vertical parallax in the image. Vertical parallax is deleted from the h1 (and the h2 which is derived from it) when a variety of vertical views is not collected. Consequently the viewer is prohibited from seeing over or under an image. The three dimensionality is retained only in the horizontal direction. Holograms which lack vertical parallax are commonly called rainbow holograms because the viewer moving his eyes vertically perceives an image which changes colors throughout the spectrum when the hologram is illuminated with a white light source. Although rainbow holograms contain images with incomplete three dimensionality, economy is realized since the requisite computed views need not span a vertical as well as horizontal range.
The making of a hologram by the procedure just described is laborious. It requires the construction of a first hologram, an h1, which is ultimately obsolete. A direct approach was introduced in U.S. Pat. No. 4,778,262 which was granted in October 1988. That technique requires no h1 construction. Each portion of the computed data is used to create a tiny elemental image-plane hologram directly. These elements are placed side by side to form the composite hologram.
This direct method can be very difficult to implement. The common methods of computer image generation must be highly modified. Otherwise their use will yield images which are unacceptably distorted. In a related process in which normal views of an image (i.e. no image-plane views) are collected, and then recomposited to form elemental views, unorthodox processing is required.